Film projectors, televisions, and computer monitors utilize an additive color model where the individual colors (Red, Green, and Blue--RGB) may be presumed pure, and therefore may be combined to produce high-quality colors in a relatively predictable manner. In contrast, color printing utilizes the concept of subtractive color in which three inks combined in different amounts absorb different amounts of the RGB colors. These inks, known as CMYK, are cyan (which absorbs red light), magenta (which absorbs green light) and yellow (which absorbs blue light). Black ink is also used to shadow areas to increase contrast and make up for deficiencies in the inks themselves. Unlike, the relatively pure RGB colors which are projected in light, CMYK color production must accommodate for ink impurities. Ink impurities prevent a direct transition from a RGB to a CMYK model. Thus, it is difficult to accurately represent on an additive color device, the effect that corrections will have on a subtractive image.
Traditionally, drum scanners have been employed to convert film into data for high quality prints suitable for use in magazines, books, brochures, and other printed material. Film is placed on the drum scanner and then treated with an oil-based composition. The drum scanner is then rotated and an optical assembly separately analyzes with photo multipliers the rotating film through red, green, and blue filters to produce CMYK color plates.
Recently, flat-bed electronic scanners utilizing charge coupled devices, or CCDs, have been employed to scan film to produce digital data which may be further processed by a host computer. Specifically, a scanner scans film through a red, a green, and then a blue filter. Red, green, and blue data of the photograph are thereby produced. Afterwards, the individual data files are interleaved by the scanner to produce a single file with a composite red, green, and blue (RGB) digital data image. This combined RGB data is then conveyed to a host computer which may project the RGB data onto a monitor. The user may make adjusts to the RGB data while viewing it on the monitor. After adjustments, the host computer converts the RGB data to individual cyan, magenta, yellow, and black (CMYK) data files. These individual data files may then be prepared for the printing press by creating separate plates from the image of the film.
This approach is advantageous since it reduces the need for intermediate photographic films and manual image assembly. The approach is also advantageous since the photographic original is scanned directly, without having to apply any materials to the photograph, as is done in drum scan technology. In addition, this approach produces digital data which may be conveniently utilized for further processing and incorporation with text.
On the other hand, this approach has not been able to produce the quality reproductions which are realized with drum scanners. One problem relates to the necessity of compressing the tonal range when one moves from film to a printed image. A transparency has a large density range, while a printing press has an extremely limited range. Consequently, tonal compression is necessary when moving from a transparency to a printed image. If tone is not compressed properly, the resultant image will lose important tonal detail or have unnatural tone shifts.
It is difficult to determine where to compress an image. Typically, RGB scanners utilize a simple compression of the overall tonal range in accordance with a gamma curve. This approach does not allow for the optimum data to be captured for specific reproduction requirements and different original images. For instance, one image might depend on accurate rendition of the shadow, with greater compromise possible in reproducing the highlights, while another image might be completely the opposite, where the highlights are more important to the picture.
Another problem with digital systems is treating color casts. An original image will often have a color cast, usually most notable in a particular area such as a highlight. These casts should be removed for successful reproduction.
Another problem with prior art printing systems relates to black generation. Black is added to achieve detail and contrast in shadow areas. Black adds the density that is lost due to additivity failure when the inks are printed on top of each other. Prior art systems are limited to black generation by skeleton black or to default black generation methods utilizing only under color removal or gray component replacement.
The problem associated with gray component replacement is that it produces an undesirable color shift. In addition, it is difficult to decide how much color to remove and how much black to replace it with.
A problem associated with all reproduction processes is the lack of sharpness in the original image. If smooth areas of a figure are erroneously adjusted for sharpness, then the texture of the area is augmented and becomes unnatural. In most image manipulation programs, sharpening, or unsharp masking, is done with filters based on a convolution matrix, by analyzing a pixel and its immediate neighbors. Because digitally scanned data is often quite coarse, single pixels can be distinctly different from their neighbors, and these filters can adversely affect smooth tones in an image. To smooth flat areas, one must distinguish between an artifact of scanned data and an actual edge. In an ideal system, one would be able to determine which areas should be smoothed, and which edges should be sharpened and by how much. Moreover, it would be highly advantageous to be able to control the adjustment of the sharpening threshold, the amount of sharpening applied, the amount of contrast at the edges, and the addition of tonal smoothness.